In a magnetic disk storage apparatus, data on the recording media is read and written by a magnetic head. In order to increase the recording capacity per unit area of the magnetic disk, it is necessary to increase the area recording density. However, the area recording density of existing in-plane recording systems can not be increased as the length of bits to be recorded is decreased because of thermal fluctuation in the magnetization of the media.
A perpendicular recording system which records magnetization signals in a direction perpendicular to a medium is adapted to address this problem. In the perpendicular recording system, a magnetoresistive head (“MR head”) or a giant magnetoresistive head (“GMR head”) with a larger read output than non-perpendicular systems can be used for reading. However, a single pole head must to be used for the writing head in these systems. With perpendicular recording, it may be necessary to improve the track density and the linear recording density in order to improve the recording density. To improve the track density, the track width of the magnetic head is decreased and formed with higher accuracy.
In a perpendicular recording system, the shape of the main pole of the single pole type recording head has a significant effect on the magnetization pattern of the media. Specifically, the shape of the upper end face of the main pole, which is the end face of the main pole on the side opposite to the MR head (on the trailing side), greatly affects the shape of the magnetization pattern of the media. For example, JP-10-320720/1998 discloses the structure of a single pole type head having a main pole of a trapezoidal shape flattened at the upper end face and wider on the side of the MR head.
However, in the description in JP-10-320720/1998, a description is made of side recording tracks defined by the slope on both sides of the trapezoidal shape. These side recording tracks reduce cross talk with adjacent recording tracks, however, they hinder the improvement of the track density which therefore hinders improvements in the area recording density. In such a magnetic disk storage apparatus, a skew angle is formed when the magnetic head scans from the inner circumference to the outer circumference of a disk, in which the pole shape and associated magnetic fields erase signals on adjacent tracks. JP-10-320720/1998 has no specific descriptions about the pole forming method.
By using a polishing method, the upper surface of the main pole (second pole) can be flattened. However, when a polishing method such as chemical mechanical polishing (CMP) is used, it is difficult to control the layer thickness which hinders the accuracy of the layer thickness. The thickness may vary by as much as about ±0.5 μm. This inaccuracy scatters the layer thickness of the main pole, thereby causing scattering in the intensity of the magnetic field from the main pole.
FIGS. 1A-1B show a prior art design of a perpendicular head 100. The performance of this design depends on the flux carrying capacity of the probe layer 102 as well as the shaping layer 104 placed underneath. The saturation point for the probe layer is located just after the end of the shaping layer. This saturation of the probe reduces the head's efficiency and the amount of flux that can be delivered to the disk. This reduced flux demands lower coercivity media thus reducing the latency of the recording operation.
FIG. 2 illustrates a prior art single layer coil head 200 where the end of the shaping layer 202 (zero throat height (ZTH)) is placed behind the ABS plane 204. This distance is controlled by two limits. One is the close-to-the-ABS limit where the shaping layer will start writing on the disk. The other limit is the too-far-from-the-ABS limit where the probe flux is very limited by saturation and overwrite problems occur.
Thus, it is desirable for a pole write head to be so designed that the flux density at the pole tip is close to the saturation flux density of the magnetic material used for the pole so that the largest possible write field may be obtained, permitting the use of high coercive field media with well known advantages in terms of thermal stability and recording resolution. When a pole head of constant cross-section is in position over a recording medium with a magnetically soft underlayer, the flux density increases with distance from the pole tip because fringing flux between the sides of the pole and the underlayer adds to the flux which comes through the air-bearing surface end of the pole. It is well known that the flux density can be decreased by increasing the cross-section of the pole with distance from the air-bearing surface. For example, in U.S. Pat. No. 4,710,838, A. Jahnke describes a “widening leg part”, and in U.S. Pat. No. 5,600,519, D. Heim and M. Williams describe how designing the cross-section area proportional to the flux density that would obtain in a constant cross-section pole can be used to extend the length of the saturating region. In U.S. Pat. No. 5,479,310, Atsushi, et al, describe a longitudinal recording head with a deliberate reduced cross section to limit saturation at the pole tips.
The manufacturing process for thin film recording heads includes a step in which individual or rows of heads are lapped to provide the correct stripe height on the sensor element which is used to read data from the disk. Because of various manufacturing tolerances, this results in some variation in the location of the air-bearing surface along the pole in the direction normal to that final air-bearing surface. As a consequence, any increase in cross-section of the pole accompanied by a variation in width at the trailing edge of the pole would result in an undesirable variation in written track-width. It is also understood that in many applications, a pole head should be capable of operating at several degrees of skew as determined by the rotary head positioning actuator design without writing on adjacent tracks, so a trapezoidal cross section of the pole is desired when viewed from the air-bearing surface.
The combined effect of this desirable trapezoidal cross section and the uncertainty of the air bearing location would again result in an uncertain written track-width if the trailing surface of the pole does not lie in a plane perpendicular to the air-bearing surface. The remaining surface which may be slanted is the leading edge of the pole. Tapering the thickness of the pole by sloping the leading edge from the air-bearing surface back to a distance such that the cross-section may be increased without danger of excessive side-writing has been shown to improve writability. In these experiments, a focused ion-beam (FIB) system was used to mill a taper from the air-bearing surface after lapping was completed. Such modified heads could write 12 KOe media, where only about 9 KOe media could be written with constant cross section poles. Similar results were reported by Y. Kawato, et al, of Hitachi, in “Single-pole type GMR heads for perpendicular recording at ultrahigh areal densities”, presented as paper CA-02 at the 8th joint MMM-Intermag conference in 2001. These experiments involving FIB milling at the air-bearing surface, however, are not regarded as economically feasible for the manufacturing production of large numbers of recording heads.